Materials Technology: Properties, Structures & Testing
1. Introduction
When choosing a material for design, manufacturing, or construction, the primary concern is matching its characteristics to the working conditions of the intended application. We must understand the properties (physical, chemical, technological, or mechanical) of the material, how to identify them, and any limitations or advantages they impose. The advancements in recent years demand materials that can withstand harsh service conditions. This necessitates careful and frequent monitoring to maintain quality and improve manufacturing processes, resulting in greater safety and economic production.
2. Basic Material Structure
The key technological properties of materials are directly related to their structure. To control these properties and utilize them effectively, it’s crucial to understand their structure. All substances consist of the same basic components: protons, neutrons, and electrons. It’s remarkable that such a wide variety of properties and behaviors arise from these fundamental building blocks. There’s a vast range of metallic and non-metallic materials, each exhibiting a unique set of properties. These properties depend on the structural levels within each material.
3. Distribution of Atoms
The arrangement of atoms in a material significantly influences its properties. Depending on how atoms are grouped, a substance can have a molecular, crystalline, or amorphous structure. In molecular structures, a specific number of atoms are linked by primary bonds, but only weakly interact with other simple atom groups. Crystalline structures are common in solid metals and minerals. Here, atoms are arranged in a geometric order known as a lattice or spatial network. In amorphous structures, like glass, atoms have some local order, but overall, their distribution is more disordered than in crystalline solids.
4. Metallic and Non-Metallic Materials
The most common classification groups materials into metallic and non-metallic. Common metals include iron, copper, aluminum, magnesium, nickel, titanium, lead, tin, and zinc, along with their alloys like steel, brass, and bronze. They all exhibit metallic properties, such as characteristic brightness, high electrical and thermal conductivity, ductility, and some magnetic properties. Well-known non-metals include wood, brick, concrete, glass, rubber, and plastics. Their properties vary widely but tend to be less ductile, tough, and dense than metals. They also lack electrical conductivity and have low thermal conductivity.
5. Classification of Properties
5.1. Physical Properties
This group includes characteristics that directly or indirectly affect our senses or define the material’s behavior under physical phenomena like electricity, magnetism, and heat.
5.1.1. Mass, Specific Gravity, and Density
Mass is a property of all matter, determined by the amount and type of particles in a body. It’s manifested by the body’s resistance to changes in motion and is constant throughout the universe.
Weight is the force with which Earth attracts a body (Fg), measured in Newtons (N). Due to this attraction, a mass of 1 kilogram has a weight of 9.81 N. Fg = mass * g (measured in Newtons), where g = 9.81 N/Kg. Gravity and weight depend on location and decrease with distance from Earth.
Density is the mass of 1 cubic centimeter of a substance. It’s the ratio of mass to volume: Density = Mass (g) / Volume (cm³).
5.1.2. Thermal Conductivity
This is the ease with which a body transmits heat energy through itself. Metals generally have good thermal conductivity, unlike other materials, which are often poor conductors. The amount of heat passing through a body of thickness (l) depends on:
- The temperature difference between the hot and cold sides.
- The surface area of the body (larger area allows more heat transfer).
- The thickness to be traversed.
- The nature of the body, specifically its conductivity.
5.1.3. Specific Heat
Defined as the amount of heat needed to raise the temperature of one gram of a substance by one degree Celsius. It varies for different substances. The coefficient of thermal conductivity is relevant in thermal treatments. To treat a metal, its temperature must be raised and lowered according to a specific program, requiring calculations of the heat amount and heating time.
5.1.4. Thermal Expansion
This property describes the increase in volume of a body when its temperature rises. In practice, we’re interested in the increase in length in one direction, represented by the coefficient of linear expansion (m). It’s defined as the increase in length per unit length of a body for a one-degree Celsius temperature increase.
5.1.5. Melting Temperature and Latent Heat
When a metal’s temperature rises progressively, it reaches a point called the melting temperature or melting point, where it changes from solid to liquid. This temperature is a well-defined characteristic of metals and roughly coincides with the solidification temperature, where the reverse change from liquid to solid occurs.
Latent heat of fusion: The amount of heat absorbed by a body to change from solid to liquid at a constant temperature.
Latent heat of solidification: The amount of heat released by a body to change from liquid to solid at a constant temperature.
5.1.6. Electrical Conductivity
This property, almost exclusive to metals, represents the ease with which a body allows electric current to pass through it. Electric current is created by the movement of electrons within a body between two points with different electrical potentials. Electrical conductivity depends on the strength of atomic bonds, which in metals is primarily due to ‘free’ electrons not involved in strong bonds. Conversely, materials that resist the flow of electric current due to their internal structure are called insulators.
5.1.7. Magnetic Properties
These are the abilities of some materials to be attracted by magnetic forces and acquire magnetism. Examples include iron (ferromagnetic), cobalt, nickel, and steel. The fact that an electric current flowing through a conductor creates a magnetic field is utilized in various electrical devices like motors, generators, transformers, and solenoids. Their cores and armatures are made of ferromagnetic materials, taking advantage of their magnetic properties.
5.1.8. Optical Properties
These properties relate to how a material interacts with light. The most obvious is transparency. A transparent material allows light rays to pass through, enabling us to see images through it. A translucent material lets some light through but not enough for clear vision. An opaque material doesn’t allow light to pass through.
5.1.9. Reflective Properties
Reflection is the change in direction of a light ray when it strikes a material. All materials reflect light, and this reflection allows us to see them. Color is a result of light reflection. While light usually behaves like a wave, it’s actually a stream of tiny energy packets called photons that act as both waves and particles. The color of a material also affects its ability to absorb and radiate heat. A dark surface absorbs heat faster than a glossy surface and also radiates heat more quickly.
5.2. Chemical Properties
Chemical processes transform substances into others with different properties. From a chemical standpoint, the most important properties are oxidation and corrosion.
Oxidation: The destructive effect of oxygen on a material’s surface. It’s a chemical reaction where oxygen combines with a substance, causing it to lose electrons.
Corrosion: Closely linked to oxidation, it’s the destructive action on metal surfaces caused by air and oxygen in the presence of electrochemical agents. The main factors contributing to corrosion are:
- The amount of water vapor and salt or acidic vapors in the atmosphere.
- The surface roughness.
- The heterogeneity of the metal due to its chemical composition and structure.
Corrosion occurs in different forms depending on the material quality and the factors involved.
Types of Corrosion:
- General Corrosion: Occurs uniformly across the metal surface, leading to reduced dimensions and weight loss.
- Localized Corrosion: Originates in galvanic couples, which are regions where the destructive effect is concentrated.
- Intergranular Corrosion: The most dangerous type, caused by the presence of cathode areas formed by impurities at the grain boundaries, leading to their destruction without significant external signs of corrosion.
5.3. Technological Properties
These properties indicate how a material behaves during processing.
- Castability: Refers to materials that can be melted and poured into molds at economically viable temperatures (e.g., cast iron).
- Malleability: Materials that can be shaped by the action of forces, accepting plastic deformation (e.g., embossing, folding).
- Machinability: Materials that can be cut and shaped by removing metal using reasonable technological forces, breaking the cohesion of particles (e.g., drilling, filing, turning).
- Weldability: Materials that can be joined by local cohesion using appropriate welding techniques.
- Hardenability: Indicates that the material’s hardness can be altered by rearranging its particles (e.g., heat treatment of metals).
5.4. Mechanical Properties
These properties describe a material’s behavior under the action of external forces. They can be classified based on the nature of the stress.
5.4.1. Resistance to Fracture and Toughness
A material’s resistance is its ability to withstand changes in shape and separation. If external forces overcome the forces of cohesion, fracture occurs. Depending on how external forces act on a piece:
- Compression: Resistance to forces that tend to compress it.
- Tension: Resistance to forces that tend to lengthen it.
- Shear: Resistance to two forces acting perpendicular to its axis in the same section, tending to cut it.
- Bending: Resistance to a torque acting perpendicular to its axis, tending to bend it.
- Torsion: Resistance to a force couple whose plane is perpendicular to its axis, tending to rotate each cross-section of the bar relative to the others.
5.4.2. Elasticity and Plasticity
Elasticity is the ability of some materials to recover their original shape after being deformed when the external force is removed. The elastic limit is the point where external forces start to cause permanent deformation. Bodies that don’t recover their original shape are called inelastic or plastic.
- Brittleness: The inability of a material to withstand impact or sudden forces.
- Resilience: The resistance of a body to impact or sudden forces. Resilience is the opposite of brittleness; higher resilience means lower brittleness.
- Hardness: Defined as the resistance of a body to indentation.
- Cohesion: The resistance of metal atoms to separation from each other, depending on how they are bonded.
- Fatigue: The ability to withstand repetitive stress varying in magnitude and direction.
6. Material Testing
Material testing verifies properties like strength, toughness, hardness, consistency, and behavior at different temperatures. It also determines chemical composition to assess purity, corrosion resistance, and workability. An important function of testing is to determine the cause of breakage or unwanted deformation during operation. Test methods can be classified as:
- Destructive: Disable the material and determine mechanical and technological characteristics.
- Non-destructive: Detect internal defects, heterogeneity, and cracks without damaging the material.
6.1. Destructive Tests
6.1.1. Tensile Tests
This test determines tensile properties by subjecting a specimen to gradually increasing tensile stress until it breaks, revealing its toughness and elasticity. Standardized specimens with circular, square, or rectangular cross-sections are used, consisting of a central body and two cylindrical or conical heads for gripping in the testing machine.
Stress-Strain Diagram: The tensile curve has two characteristic regions. The initial straight line (OA) represents elastic deformation, and the ordinate at point A indicates the apparent elastic limit (yield point). The OA region follows Hooke’s law, showing proportionality between stress and strain. The elastic elongation is determined by Oa. The BCE region represents permanent or plastic deformation. From E onwards, elongation increases even with decreasing load, and necking occurs until the actual break at C. Point B corresponds to the maximum load or ultimate tensile strength, given by OR. This is distinct from the fracture load at C, which causes the actual break.
6.1.2. Compression Test
The specimen is subjected to a slowly increasing compressive load. To calculate the compressive strength (Rc), the force (F) at the first crack is divided by the cross-sectional area. The stress-strain diagram is similar to that of tension, with data of opposite sign. In elastic materials, there’s no true compressive breaking load, as they crush without breaking.
6.1.3. Shear or Cut Test
This test determines the shear strength of materials. A double-edged shear is used to avoid bending stresses. The stress-strain diagram is similar to tension and compression, with a proportionality zone, yield point, elastic region, and breaking point.
6.1.4. Bending Test
Verifies a material’s ability to bend at room temperature. Flat bars (30-50 mm wide) or round bars are commonly used. The specimen is slowly and uniformly bent around a mandrel until the required angle is achieved. No cracks should appear on the outer surface of the bend.
6.1.5. Impact Bending Test
Evaluates a material’s behavior under impact by notching a specimen and striking it with a pendulum hammer. It’s a dynamic, not static, test. The test is performed on a pendulum device where a hammer-like pendulum hits the center of a specimen supported at two points. The resilience is indicated after the impact. Impact bending tests are conducted on steel and steel castings to determine toughness and deformability, calculate and control aging, and monitor heat treatment processes. Tough materials require high resilience, while brittle materials require low resilience.
6.1.6. Fatigue Test
Machine parts are constantly exposed to alternating loads, leading to fatigue signs after prolonged use. This can cause fatigue cracks even without exceeding the allowable stress based on strength calculations. A material is fatigue-resistant if it withstands a certain number of load variations. Testing machines allow for fatigue tests under flat bending, axial stress, torsion, and rotating bending. The fatigue curve’s average stress varies with the test type but has a basic similarity. It usually has an asymptote parallel to the abscissa, and its ordinate corresponds to the fatigue limit. Rotating bending and torsion fatigue tests are the most common.
6.1.7. Hardness Test
Various methods measure a material’s resistance to indentation, determining its hardness. Common methods include Brinell, Vickers, Rockwell, and rebound tests. In indentation methods like Brinell and Vickers, hardness is measured by the penetration depth of a body under a specific and constant pressure. The rebound method measures the height of the elastic rebound of a hard body dropped from a specific height onto the material.
Brinell Test: A steel ball of diameter D is pressed into the flat surface of a metal under a pressure P. The resulting indentation diameter depends on the pressure and the material’s resistance to penetration (hardness).
Vickers Test: Similar to Brinell, but uses a diamond pyramid with a square base and a 136° apex angle instead of a steel ball. It’s suitable for materials with hardness greater than 500 HB.
Rockwell Test: Unlike Brinell and Vickers, which require microscopic measurement, Rockwell measures the depth of the permanent indentation produced by a specific load on a diamond indenter with a spherical tip (for hard materials) or a spherical indenter (for soft materials).
6.1.8. Drawing Test
Determines the suitability of plates for deformation by drawing. The Erickson method is commonly used, and the degree of drawing is indicated by the depth of the first crack in millimeters.
6.1.9. Metallographic Test
The specimen is ground, polished, and etched with an acid. The treated surface is observed under a magnifying glass or microscope. At high magnification, the structure, cracks, and rolling direction become visible.
6.1.10. Chemical Analysis
Provides an accurate understanding of a material’s composition, including the type and quantity of alloying elements. This requires laboratory analysis.
6.1.11. Spectrographic Test
Determines the composition of materials, including the type and quantity of alloying constituents, by using a spectrograph. An electric arc is created between an electrode and the specimen, and the emitted light is decomposed through a prism in the spectrograph. The resulting spectrum reveals the metal’s composition.
6.1.12. Other Tests
Other tests include fracture tests, double folding, creasing, alternating stress tests, etc.
6.2. Non-Destructive Testing
These tests detect defects in a material (cracks, blowholes) without damaging the piece.
6.2.1. Magnetic Methods
When a piece is magnetized with a sufficiently uniform field, surface cracks, shrinkage, or blowholes cause disturbances in the magnetic field around them. These disturbances depend on the crack’s position and orientation relative to the field.
6.2.2. Electronic Methods
Based on variations in resistance caused by impurities in a conductive metal. An electric field is established in the metal through two contacts, and two probes connected to headphones explore the surface. Variations in the field are measured with a microvoltmeter.
6.2.3. Surface Tension Penetration Methods
The piece is immersed in a liquid that penetrates surface defects (cracks, porosity). The surface liquid is then removed, and the liquid remaining within the defects is detected by methods like oil exudation.
6.2.4. X-ray Methods
Based on the absorption of X-rays as they pass through the material. If a piece of uniform thickness with internal holes or heterogeneity is irradiated with X-rays, and a photographic plate is placed behind it, the areas with defects will appear darker on the plate due to greater absorption.
6.2.5. Gamma-ray Methods
Gamma rays are similar to X-rays but have a much shorter wavelength. The piece is subjected to gamma radiation, which, after passing through, creates an image on a photographic plate.
6.2.6. Ultrasonic Methods
Ultrasound consists of elastic vibrations similar to sound but with a higher frequency. This method relies on the fact that ultrasonic waves propagating through a material are reflected when they encounter even the smallest crack, air pocket, or other obstacle.
Bibliography
- Cevera Ruiz, M. and Blanco Díaz, E.: Fundamentals of Mechanics of Materials and Structural Design. 1999.
- Garrido Garcia, J.A. and Foces Mediavilla, A.: Resistance of Materials.
- Ramirez Gómez, F. et al.: Non-destructive Tests for Quality Control of Materials. 1999.